We just used a few cabs for our last trip as it was a short trip and we didn't want to waste a lot of time waiting...especially since the MK deluxe resorts all share. We factored this into our budget before hand.
Just to give you an idea of fares, we generally were at $16-17 dollars from GF to HS with our largest tab from TTC to Animal Kingdom which was just over $20. That was a a little higher because the fares raise not just by mileage but by time in certain cases and there was a few minutes wait at AK gates to get in. We also do tip a few dollars extra...not sure if you need to or not, we just do. If you would also, make sure to factor that in. So basically if it was $16-17 and change, we would give a $20.
Since we do not rent a car, if I was in a bind to make an ADR, I would certainly put $20 aside to hail a cab again. It really was a lot easier and worth the money when sharing a resort bus and to not stress over making an ADR time. We had that experience once and it was not fun...and that was being at the WL boat dock at 7:30 for an 8:25 res at CRT. And we were still late cause the boat never came! We grabbed a CR boat at 8:15 and had to run from CR to CRT in August heat. My hubby still hasn't forgiven me for that one! Lesson learned!